Effect of heavy metals on rice irrigated fields with waste water in high pH Mediterranean soils: The particular case of the Valencia area in Spain

Waste water is widely used for irrigation purposes in agriculture and in the case of rice it is not only a normal but a frequent practice, due to the large water demands of this crop. However, this highly beneficial system, due to its use of such a valuable resource, is not entirely free from risks, one of which is that recycled water may contain small quantities of heavy metals whose accumulation could prove toxic to the environment, animals and humans. Heavy metals are usually found in higher concentrations in recycled urban and industrial waste water than in normal surface or ground irrigation water. They may not only cause toxicity problems in plants, but also in animals and in the people that consume them, or in the people that consume the animals that feed on the plants, with the associated risk of bioaccumulation, as in the case of boron, which can cause blindness. Elements that can have toxic effects on plants and animals in small concentrations include: boron, cadmium, copper, nickel, molybdenum, zinc and iron. Recycled irrigation water from urban areas does not usually contain an alarming amount of metals, except in cases in which waste water is collected from certain industries. This study focuses on the evolution along an irrigation line of the effects of using recycled urban waste water (with secondary treatment) on rice crops and on the cultivation medium. Its novel contribution as regards previous research on the use of urban waste water in agriculture and its effects on crops and the environment lies in the fact that we not only followed the time evolution throughout various crop cycles, but also the linear (spatial) evolution of concentrations in specific chemical parameters of the land flooded to grow rice, mapping the changes with the help of geographical information systems. The study included the analysis of the evolution of five heavy metals (B, Cu, Cr, Ni and Zn) in soils and rice crops on five farms along an irrigation line throughout three crop cycles. In alkaline soils such as that involved in this study, the metals are in general retained in the soil and do not pass into the plants to cause toxicity problems, but even so, with continuous long-term irrigation heavy metals can accumulate, and in the case of rice this situation is likely to arise. In the study we always considered the possibility that this crop is a heavy metal filter that prevents toxic elements from reaching the consumer.